-LRB- CNN -RRB- -- Authorities found four human bodies abandoned in the Arizona desert Thursday .

Initial indications point to exposure as the probable cause of death , said Victor Brabble , a spokesman for U.S. Customs and Border Protection .

`` It is probable that they are immigrants attempting to cross into the U.S. , '' he said . `` However , we do n't have enough to draw a conclusion on it now . ''

The bodies were found near Gila Bend , Arizona , about 70 miles north of the border . They have not been identified , the Maricopa County Sheriff 's Office said .

Immigrants often try to cross the Sonoran Desert 's harsh terrain , and there are many heat-related deaths , Brabble said Thursday .

Immigrant rights advocates have warned that even as border crossings decrease , deaths are on the rise as increased border security forces people to choose more dangerous crossing routes . Border patrol officials have argued that more security is necessary to stop smugglers .

According to a recent study by the Binational Migration Institute at the University of Arizona , more than 2,230 migrants have died in the state 's desert area along the border in the past 22 years .

In the border region of Pima County , Arizona , deaths of unidentified migrants in the desert have become so common the Medical Examiner 's Office has helped create a website to track the deaths and assist family members searching for their loved ones ' remains .

On Thursday , a bipartisan group of senators announced a proposal to add 20,000 more border agents , complete 700 miles of fence along the boundary with Mexico , and deploy $ 3.2 billion in technology upgrades similar to equipment used by U.S. forces in Iraq and Afghanistan .

The proposed amendment , negotiated by a group of senators from both parties known as the `` Gang of Eight , '' is intended to ensure Senate passage of a major immigration reform bill with enough Republican support to persuade the GOP-controlled House to also take up the measure .

Journalist Valeria Fernandez and CNN 's Tom Cohen , Dana Bash and Ted Barrett contributed to this report .

@highlight

A border patrol spokesman says it 's probable they are immigrants

@highlight

The bodies have not been identified , the Maricopa County Sheriff 's Office says

@highlight

Activists say border deaths are on the rise as security forces more dangerous crossings

@highlight

Border patrol officials have argued that more security is necessary to stop smugglers